Recombinant DNA cloning techniques are being used to isolate Drosophila muscle genes which are coodinately expressed during skeletal muscle differentiation. Our objectives are to compare the structure, chromosomal locations and expression of these muscle genes in wild type flies and muscle mutants to elucidate the molecular genetic mechanisms which coordinate expression of these genes during the myogenesis. During the current year we have isolated recombinant genomic clones of two groups or coordinately expressed Drosophila muscle genes. The first group of genomic clones includes the muscle gene(s) encoding the contractile protein, myosin heavy chain. These myosin heavy chain genes were mapped by in situ hybridization to a single chromosomal locus at 36B(2L). This site is close to or within a genetic region which includes 4 genes involved in expression of filght muscle myofibrillar proteins. The second group of genomic clones includes the genes for 3 sequence-related muscle mRNAs. These mRNAs code for fusion-specific proteins with molecular weights of about 22,500 daltons. Clones of this group of muscle genes were mapped by in situ hybridization to a chromosomal locus at 87B(3R). We are currently characterizing the sequence organization and developmental expression of muscle genes at the 36B and 87B chromosomal loci of wild-type Drosophila and flightless muscle mutants.